The present invention involves an improved chopper for chopping continuous or very long loose items such as fiber, fiber strands, yarn, wire, string, ribbon, tape and the like by pulling the item(s) into the chopper while the loose items are held tightly against the surface of a rotating backup roll with a rotating idler roll biased against the backup roll and carrying the item(s) on into a nip between a rotating blade roll and the rotating backup roll where they are separated into short pieces. More specifically the present invention involves a chopper having an improved mechanism for biasing the backup roll and the blade roll against each other during the chopping operation.
It has long been known to chop continuous fibers or fiber strands into short lengths of about 3 inches or shorter. Billions of pounds of such product including chopped glass fibers and fiber strands are produced each year in process and chopping apparatus such as disclosed in U.S. Pat. Nos. 5,970,837, 4,398,934, 3,508,461, and 3,869,268, the disclosures of which are incorporated herein by reference. The choppers disclosed in these patents comprise a blade roll containing a plurality of spaced apart blades for separating the fibers into short lengths, a backup roll, often or preferably driven, which the blades work against to effect the separation and which pulls the fibers or fiber strands and an idler roll to hold the fibers or fiber strands down onto the surface of the backup roll. In the chopped fiber processes disclosed in these patents, the chopper is usually the most productivity limiting equipment in the processes. These processes typically operate continuously every day of the year, 24 hours each day, except during furnace rebuilds every few years. Therefore, improvements in the chopper, which allow the chopper to pull and chop faster and for longer times between maintenance shutdowns, and/or to pull and chop more fibers or fiber strands at a time, have an extremely positive impact on productivity and production costs.
In the prior art, the backup roll has been mounted and held against the surface of the blade roll in a generally rigid manner such as with a mechanical screw jack and a gear head stepping motor or with a variable force such as a force applied by an air or hydraulic cylinder. A shear pin or equivalent has also been used as a safety feature in the event a thicker stream of fiber strands comes to the chopper, but when the shear pin fails, considerable down time is incurred and production is lost while the shear pin is replaced and the chopper is put back on line.
The mechanical jack was set up by manually running the gear motor to bias one of the backup roll or blade roll into the other roll until the blades had penetrated the working layer of the backup roll an appropriate amount. If the blades did not penetrate far enough, double cuts or stringers, long strands, would result, an unacceptable result. If the blades penetrated too far, the chopper would chop the strands properly, but the backup roll life would be shortened substantially. Given these options, at least some operators tended to run the jacuator too long in setting up a rebuilt chopper, or if a chopping problem developed, thus reducing backup roll life substantially below what it could be if the choppers are set up properly. This is a costly situation causing this system to be abandoned in favor of using fluid cylinders with or without shear pins.
Normally several strands such as up to 14 are fed into the chopper, each strand containing 2000 or more fibers. As more fiber strands and fibers are fed into the chopper it becomes more difficult to pull all of the strands and fibers at the same speed, so more pressure is applied to the cylinder pushing the idler roll against the backup roll with more force.
Occasionally a glass bead from a fiberizing bushing or a wad of fibers will be pulled to the chopper caught up in the multitude of fiber strands. When this happens, it is necessary for one of the backup roll or blade roll to be able to move away from the other roll to allow this thicker anomaly to pass through the nip between the blade roll and the backup roll. If this separation does not occur the chopper will often lock up causing damage to the drives, belts and/or the rolls.
Although at least one of the rolls is held in position with a fluid cylinder, the fluid is either not compressible or responds too slowly to the sudden problem to protect the chopper from damage and downtime. In the past the shear pin was used to provide such protection. However, when the shear pin shears the blade roll and backup roll are no longer biased together properly requiring that the chopper be shut down to install a new shear pin. This downtime is costly because of the loss of production during the downtime and due to reduced material efficiency for several minutes following restart. Downtime causes forehearth and bushing temperature upsets because hanging fibers do not pull in cooling air that occurs when the chopper is pulling the fibers from the bushings.
If all of the strands or fibers are not pulled at the same speed, the slower strands and fibers will have a greater fiber diameter which is unacceptable and the bushings of the slower strands frequently will not operate at the proper temperature causing more frequent breakouts and/or additional fiber diameter variations, both of which are unacceptable. Also, fiber slippage can cause some of the fibers to be cut to shorter lengths than desired resulting in an unacceptable product. Therefore, it is very important that the biasing force between the blade roll and the backup roll remain proper and essentially constant.
As the pulling speed is increased, and/or as the number of strands and fibers are increased, above about 3000–4000 ft./min. (FPM), depending on the product, the present state of the art choppers begin to vibrate and the idler roll begins to allow one or more of the strands to slip some thus reducing the pulling speed of one or more of the strands. Also, if all of the strands are not pressed between the idler roll and the elastomer layer of the backup roll, a strand can slip partially out of the nip leaving some of the fibers unchopped, producing double cuts and stringers in the chopped product and causing the product to be scrapped.
U.S. Pat. No. 3,731,575 teaches an air cylinder with an adjustable stop to bias the blade roll against the backup roll so that the blades penetrate the backup roll the desired distance and no further. However, with this arrangement, the pressure in the cylinder increases when a wad or bead or other thicker strand set passes through the chopper and forces the backup roll to back away from the blade roll. Also, an air cylinder bias is subject to permitting vibration at high speeds and is therefore not desirable. Finally, this system suffers the same problem as the mechanical jack system in that it requires an operator to set the mechanical stop limiting the distance the blades can penetrate the working layer of the backup roll.
It would be very desirable for the chopper to have an adjustable, but constant biasing force between the backup roll and the blade roll while having the ability to instantaneously respond to a substantially thicker feed of material to be chopped without requiring any downtime or without causing unnecessary scrap.